If a worm sticks out its head from one of the two passages [i.e. the front or the rear], and goes back in before detaching, there are two opinions regarding invalidation of ablution, related by Al-Mawardi, Al-Ruyani, Al-Shashi, and others; the more correct of the two is that it is invalidated by it coming out; the second is that it is not, since it did not detach; but Allah knows best.